A voltage mode buck converter is used to regulate the level of a direct current (DC) voltage and maintain the regulated voltage stably at a desired level, by switching a high side switch and a low side switch to generate voltage pulses through a low-pass filter including inductor and capacitor to generate a DC output voltage. An assortment of appliances has been specified in their consumed powers under stand-by modes in an energy saving (Green Power) program driven by some advanced countries. By analyzing the power consumptions specified in stand-by modes, it was clearly found that the switching loss of the power elements such as power MOSFET in a voltage mode buck converter takes the most percentage of the total power consumption. To reduce this power loss, it has been proposed an operational mode not at constant switching frequency, but depending on the loading when it is operated under a light load such as stand-by mode, which is referred to as discontinuous current mode (DCM). For example, in U.S. Pat. No. 5,568,044 issued to Bittner et al., a voltage regulator is proposed to operate in either pulse width modulation (PWM) or pulse frequency modulation (PFM) mode, by using a PFM control circuit to switch the voltage regulator to PFM mode in which the switching frequency of the high side switch varies whenever the output current is lower than a threshold.
In addition, a voltage mode buck converter is often operated under a heavy load in typical applications, and it is therefore required an over-current protection. Particularly, for those applications of power conversion from high voltage to low voltage and high speed switching, sensing the current flowing through the high side switch by conventional methods has been hard to precisely detect the over current. For example, for an input voltage of 12V, a desired output voltage of 1.2V, and a switching frequency of 500 kHz, the duty cycle will be 0.1, and the corresponding conductive period of the high side MOS will be as short as 200 ns. Owing to such short conductive period, the noise caused by inductor, capacitor and load variation would bring the over-current protection with abnormal detection.
Further, conventionally, the switching frequency of the high side and low side switches is regulated by sensing the output current of the converter, and the over-current protection is accomplished by sensing the current flowing through the high side switch. As a result, to simultaneously implementing the over-current protection and a function of switching the converter between continuous current mode (CCM) and DCM mode, at least two pins are required for a controller chip in the converter to sense the output current and high side current respectively. If the over-current protection and the mode switch function could be simultaneously accomplished by only one single pin, the controller chip will be further shrunken, and the cost will be reduced accordingly.